The United States and Japan will step up their defence cooperation to deal with the threat from nuclear-armed North Korea as tensions in East Asia remain high, officials from the two allies said on Thursday.

Baby killer McConnell back in Australia

An Australian woman who drowned her two young sons in a bathtub in Canada has arrived back in Sydney but could face extradition if an appeal for a stiffer sentence succeeds.

Allyson McConnell, 34, flew into Sydney Airport from Vancouver on Wednesday, accompanied by her mother Helen Meager, after being released from custody in Alberta on Friday.

She refused to answer reporters' questions as she pushed a luggage trolley to the airport car park, staring blankly ahead as a female companion repeatedly said "no comment".

About five police officers followed the media pack but did not attempt to speak with McConnell who, despite being sentenced to six years, was deported from Canada after serving 10 months in a psychiatric hospital.

She is expected to stay in Gosford on the NSW central coast where her mother lives.

Her former husband, Canadian Curtis McConnell, prosecutors and the Alberta justice minister all fought to keep McConnell in Canada until the appeals against her sentence and acquittal on second-degree murder charges were heard.

Alberta politicians and their federal counterparts blamed each other for her release and exit to Australia before the appeals process had concluded.

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Alberta's Justice Minister Jonathan Denis has vowed to extradite McConnell from Australia if an appeal for a stiffer sentence is successful.

Winning her extradition, however, could prove a lengthy court process.

A spokeswoman for Australian Attorney-General Mark Dreyfus told AAP the government did not disclose extradition requests until a person was brought before a court in response to an application.

McConnell's lawyer Peter Royal said Denis has made inflammatory comments about his client and although he had almost six months notice that she was going to be deported, did nothing about it.

"We fail to see how this could have taken the minister by surprise," he said in a statement to Canadian media.

But Canada's assistant deputy minister of justice Greg Lepp said there was nothing the province of Alberta could have done to prevent McConnell's release before the appeals were heard.

McConnell admitted drowning her sons, two-year-old Connor and 10-month-old Jayden, in a bathtub in her adopted home town of Millet, Alberta, in February 2010.

At her trial last year, she was found guilty of two counts of manslaughter, after being charged with second degree murder.

Mr McConnell's family said in a statement released on Sunday: "We fear that if Allyson Meager McConnell is deported to Australia, we will never see her face justice for the horror and terror she inflicted on two innocent babies before killing them."

In sentencing McConnell, the judge found she was suffering psychological issues and there was reasonable doubt she had the specific intent to kill Connor and Jayden.

Her trial heard the McConnells' marriage had broken down in 2009, Mr McConnell moved out of the family home and filed for divorce and a judge ordered McConnell could not take her sons back to Australia.

Mr McConnell found his two sons floating in the bathtub, with his former wife's wedding ring sitting on the toilet seat next to the bath.